In all radio receivers, the first amplifier after the antenna when entering the receiver should be especially low noise type, because the signal level at the input of this amplifier is very low, and the additional noise caused by the amplifier is amplified in all the following amplifier stages. An abbreviation LNA is used of such a low-noise preamplifier. Some allowed maximum value is generally specified in receivers for the total noise figure of the LNA and its input and output circuits. Losses on the transmission path cause signal attenuation, which directly increases the noise figure by the same amount. Hence, for example, if the antenna filter of the receiver is very low loss, the noise figure of the LNA can be correspondingly a little higher.
FIG. 1 shows a block diagram of the common structure of the antenna side part of a receiver. In addition to the antenna and a possible antenna switch, the structure includes an antenna filter and an amplifier unit AU. In the example of the figure, the antenna filter RXF has two parts: Starting from the antenna, there is first a bandpass filter 110 and then a low-pass filter 120. These filters can form a mechanically integrated structure. The former attenuates frequency components outside the receiving band of the radio system, and the latter further cleans up the area above the reception band. The amplifier unit AU has two parallel amplifier branches. For this, the signal Ein coming from the low-pass filter 120 is divided into two identical parts E11, and E21 in the divider 130. The phase of the first division signal E11 is changed 90 degrees in the phase shifter 140 and then amplified in the first LNA 150. The phase shifter gives a delayed signal E1p, and the first LNA gives the signal E12. The second division signal E21 is amplified in the second LNA 160, and the phase of the signal is then changed 90 degrees in the second phase shifter 170, which gives the signal E22. Again, the in-phase signals E12 and E22 are summed in a combiner 180, the output signal of which, Eout, continues towards the mixer of the receiver. Compared to a single LNA, the impedance matching of the amplifier unit described above is easier, especially towards the antenna filter. In addition, a wider dynamic and linear area and a better stability are achieved. On the other hand, the divider, the phase shifter and the additional wiring required by them cause more attenuation in the signal, which directly impairs the noise figure of the amplifier branch.
FIG. 2 shows an example of a known arrangement according to FIG. 1 for dividing the received signal before amplification. It comprises a circuit board 101, the lower surface of which, not visible in the figure, is conductive and functions as the signal ground GND. The integrated antenna filter RXF comprises resonators, and its output is connected through a connector 125 on its end wall to a coaxial cable 129, which has a characteristic impedance of 50Ω. The conductive cable sheath is connected to the signal ground at both ends. The cable 129 continues on the circuit board 101 as a transmission line, which consists of a strip conductor 131 on the upper surface of the board, a ground conductor on the lower surface and dielectric material between them. The transmission line is dimensioned so that its characteristic impedance is 50Ω. It belongs to the divider 130 as its input line. The divider is of the Wilkinson type, which means that the input line mentioned above branches into two transmission lines, the length of which on the operating frequency is λ/4 and the characteristic impedance √2·50≈71Ω. One of the two transmission line branches is formed of the first division conductor 132 on the upper surface of the board, a ground conductor on the lower surface and dielectric material between them, and the second branch correspondingly of the second division conductor 133 on the upper surface of the board, a ground conductor on the lower surface and dielectric material between them. A Wilkinson divider is formed when the tail ends of the first and the second division conductor have been connected together by a resistor 134 of the size of 2·50=100Ω. In that case, if both transmission line branches have been terminated by an impedance of 50Ω, the energy coming from the filter is divided into them half and half, and theoretically without losses. Thus, the divider does not consume energy in spite of the resistor 134 in it. Only if the matching on the transmission paths continuing forward is inadequate, the resistor 134 causes losses. In addition, a good isolation between the branches is achieved. The phase shifter 140 in FIG. 1 has been implemented with a quarter-wave long transmission line, of which the strip conductor 141 as the continuation of the first division conductor 132 is seen in FIG. 2. This terminates at the input pin of the first LNA 150. The second division conductor 133 terminates directly at the input pin of the second LNA 160.
The arrangement according to FIG. 2 has the drawback of losses that occur in it in practice: the circuit board material causes dielectric losses in the divider 130 and in the phase shifter 140, the size of the losses being typically 0.2-0.5 dB in the former and 0.1-0.3 dB in the latter. The transmission line 129 from the filter to the divider and its connectors cause more losses, the size of which can be several tenths of a decibel, naturally depending on the length of the line. These attenuations directly increase the noise figure of the amplifier unit by the same amount. Then the requirements for the LNA itself correspondingly increase if the total noise figure must remain as low as possible.